This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-051878, filed Feb. 27, 2002, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an X-ray computer tomography apparatus having a function of executing a pre-scan before a main-scan.
2. Description of the Related Art
In contrast-medium imaging, it is important to start a scan (main-scan) at the timing a contrast medium injected into an object to be examined flows into a region of interest. For this reason, a scan (pre-scan) is executed before a main-scan. In order to suppress the x-ray dosage, a pre-scan is executed by using X-rays lower in intensity than those used in a main-scan. The projection data acquired in a pre-scan is supplied for real-time reconstruction processing. The image (tomographic image) reconstructed in real time is instantly displayed. An imaging technician visually checks the contrast medium concentration (the degree of staining) of a region of interest on the image. Upon determining that the contrast medium concentration of the region of interest has reached a predetermined concentration, the imaging technician presses a trigger button for a main-scan. When the trigger button is pressed, a pre-scan is stopped, and a preparation (setup) for a main-scan is started. The preparation for the main-scan includes the operation of moving the couch into a predetermined scan range, the operation of accelerating the couch to a predetermined speed, the operation of accelerating the rotating frame to a predetermined scan speed, the operation of heating the filament, and the like. Upon completion of the preparation for the main-scan, the main-scan is started.
A technique called bolus tracking method has been developed, in which imaging technician""s operation of pressing the trigger button is automated. In bolus tracking method, the CT value or average value of a region of interest is extracted from an image obtained by a pre-scan and compared with a threshold. When the CT value or average value of the region of interest exceeds the threshold, the pre-scan is stopped, and a preparation for a main-scan is started.
In X-ray computer tomography apparatuses, an ECG gated scan scheme of obtaining one or a series of images in a specific time phase by performing a scan in synchronism with an electrocardiographic wave is known. According to this scheme, in order to reduce motion artifacts due to the movement of the heart, the time (scan speed) required for the X-ray tube to rotate once is required to be sufficiently short compared with the cardiac cycle.
In order to maintain a high diagnostic value by reducing motion artifacts to a certain degree, a scan speed as short as, for example, 500 msec is required. It is not easy to realize a condition of a scan speed of 0.2 sec.
As a technique of reducing motion artifacts in a relatively popular apparatus whose scan speed is 1 or 0.5 sec, a technique called ECG gated reconstruction (segment scan) has been developed. In this segment scan, electrocardiogram data is acquired along with acquisition of projection data. On the basis of the electrocardiogram data, data (segment) in a partial period centered on a specific cardiac time phase is extracted from a projection data set in each cardiac cycle. A plurality of segments extracted from a plurality of cardiac cycles are joined to each other to prepare 360xc2x0 projection data necessary for image reconstruction.
In a segment scan, the number of segments to be acquired is often set in advance. As the time width of a segment decreases, the number of segments required to be acquired increases, and the time resolution of an image increases. That is, the number of images in different cardiac time phases increases. In contrast to this, as the time width of a segment increases, the number of segments required to be acquired decreases, and the time resolution of an image decreases. If the number of segments acquired does not reach the predetermined number, an image with a predetermined time resolution cannot be obtained. In order to obtain an image with the predetermined time resolution, at least projection data corresponding to a heart rate corresponding to the predetermined number of segments to be acquired is required.
A segment scan is susceptible to the influences of motion artifacts. For this reason, this scan is performed while the patient holds his or her breath. There is a limit to the time during which the patient can hold his or her breath. In a segment scan, necessary projection data corresponding to the heart rate must be acquired within the limited breath holding period. In the segment scan, therefore, the heart rate of an object to be examined must be measured in advance, and the scan speed, i.e., the time required for the X-ray tube to rotate once, must be adjusted in accordance with the heart rate.
As described above, in bolus tracking method, a pre-scan is executed before a main-scan. When the segment scan scheme is applied to a main-scan, the time difference between the instant at which a scan speed is set and the instant at which a main-scan is started becomes long. Therefore, the heart rate in the main-scan may greatly change with respect to the heart rate at the time of setting the scan speed. For this reason, in some cases, the scan speed in a main-scan is not appropriate with respect to the actual heart rate. This problem becomes more noticeable in a contrast medium examination. This is because the heart rate often greatly changes due to the injection of a contrast medium.
It is an object of the present invention to allow scan condition reset for a main-scan with respect to a change in physiological phenomenon of an object to be examined in a bolus tracking method by an X-ray computer tomography apparatus.
According to a first aspect of the present invention, there is provided an X-ray computer tomography apparatus which executes a pre-scan to acquire data for an object injected a contrast medium by using relatively low X-rays, and executes a main-scan to acquire data for the object by using relatively high X-rays after the pre-scan, comprising:
a determination unit configured to determine, on the basis of data of a physiological phenomenon of the object measured in a period between the start of injection of the contrast medium or the pre-scan and the end of the pre-scan, necessity of resetting a scan condition for the main-scan which is initially set on the basis of data of the physiological phenomenon of the object measured before injection of the contrast medium and a scan condition setting unit configured to reset, when the determination unit determines that it is necessary to reset the scan condition, the scan condition for the main-scan from the initially set scan condition to a scan condition corresponding to the data of the physiological phenomenon measured in the period between the start of injection of the contrast medium or the pre-scan and the end of the pre-scan.
According to a second aspect of the present invention, there is provided an X-ray computer tomography apparatus which executes a pre-scan to acquire data for an object injected a contrast medium by using relatively low X-rays, and executes a main-scan to acquire data for the object by using relatively high X-rays after the pre-scan, comprising a scan condition setting unit configured to set a scan condition for the main-scan on the basis of data of a physiological phenomenon of the object measured in a period between the start of injection of the contrast medium or the pre-scan and the end of the pre-scan.
According to a third aspect of the present invention, there is provided an X-ray computer tomography apparatus which executes a pre-scan to acquire data for an object injected a contrast medium by using relatively low X-rays, and executes a main-scan to acquire data for the object by using relatively high X-rays after the pre-scan, comprising a display unit configured to display the data of a physiological phenomenon of the object in time graph and numerical value and also display an input field concerning a scan condition for the main-scan, an input unit configured to input a scan condition for the main-scan in the input field, and a scan condition setting unit configured to set a scan condition for the main-scan in accordance with a scan condition input in a period between the start of the contrast medium or the pre-scan and the end of the pre-scan.
Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.